In high-strength fastening parts used for automobiles, ordinary machines, and buildings, a high-strength alloy steel with increased content of Cr, Mo and so on is used and subjected to quenching and tempering treatments, thereby ensuring a target strength. In high-strength fastening parts used for buildings or various light electrical appliances, a low carbon steel with carbon content of about 0.20% is usually used and subjected to carburizing, quenching, and tempering treatments, thereby ensuring a target strength.
However, in the former case, effect of hydrogen penetration into the steel in use environment may cause fracture (delayed fracture) of the bolt after tightened, and therefore the actual tensile strength of the bolts is restrained to 1100 MPa or less. In the latter case, carburizing and quenching cause a topmost surface hardness of more than Hv 600 (1960 MPa in terms of tensile strength), increasing sensitivity of the bolt to a slight environmental change, such as dew condensation due to temperature difference, so that the bolt involves a risk of delayed fracture.
Since it is assumed that the delayed fracture is caused by complicatedly interwound factors, it is difficult to specify a cause therefor. However, it is generally common understanding that hydrogen embrittlement is related to the delayed fracture. As factors affecting the hydrogen embrittlement, the following have been provisionally recognized: tempering temperature, structure, material hardness, crystal grain size, various alloy elements, and others. However, a process for preventing the hydrogen embrittlement has not been established and therefore the fact is that various processes based on trial and error have been suggested.
In recent years, attention has been paid to non heat-treatment bolts to which quenching and tempering steps after bolt forming have been omitted, in view of decreasing costs for the bolt manufacture as well as decreasing greenhouse gas emission during the process for the bolt manufacturing. The non heat-treatment bolts are required to ensure a target strength by work hardening during the wire drawing. However, cold-forging of the work-hardened steel wire causes restriction on the bolt shape and shortening the forging die life. Since this effect becomes more remarkable as the bolt strength is made higher, a solution of the deleterious effect has been strongly desired. Against these problems, the following processes are disclosed as conventional arts.
Patent Document 1 discloses a technique of making use of the dispersion of fine compounds to restrain delayed fracture. In this technique, an alloy steel is subjected to quenching and then subjected to tempering at high temperature so as to precipitate a large amount of fine alloy compounds and further causing the resultant precipitation to trap hydrogen moving around in the steel (diffusible hydrogen), thereby improving the steel in delayed fracture resistance. However, this solution essentially requires addition of a large amount of alloying elements and steps for the quenching and tempering, causing problems that costs for manufacturing bolts are increased and that greenhouse gas is released into the atmosphere when the bolts are manufactured.
Patent Document 2 discloses a process of subjecting perlite steel to high strength wire drawing to manufacture a non heat-treatment bolt improved in delayed fracture resistance. In this technique, formation of perlite structure vanishes prior austenite grain boundaries largely lowered in grain boundary strength by hydrogen embrittlement and further cause an interface between cementite and ferrite in the perlite structure to trap hydrogen in the steel, thereby improving the steel in delayed fracture resistance. However, in the technique disclosed in Patent Document 2 targeting a bolt strength of 1500 MPa, the high proportion of the perlite structure resulting from prioritizing the high strength causes the high proportion of the perlite structure so that the forging die life is greatly shortened due to increased deformation resistance during bolt manufacturing.
Patent Document 3 discloses a technique for improving delayed fracture resistance of a non heat-treatment steel for upset bolt with a tensile strength of 900 MPa or more by dispersing a precipitation in ferrite and perlite structures. However, in case that the tensile strength of a bolt is 1100 MPa or more, the critical upset ratio to crack initiation is lowered as the steel is subjected to high strength wire drawing. This causes generation of crack and lowering in delayed fracture resistance during bolt manufacturing.
Patent Document 4 discloses a technique for improving cold forgeability of non heat-treatment steel for bolt with a tensile strength of 900 MPa or more by using bainite structure. However, since the bainite structure is low in work hardening rate, a bolt strength of 1200 MPa or more can not be easily achieved. Further, the bainite structure is more easily affected by stress relaxation due to relaxation-operation than martensite and perlite structures. As a result, a problem is caused also from the viewpoint of the maintenance of properties of the bolts after tightening the bolts.
Patent Document 5 discloses a technique for yielding a non heat-treatment steel wire for bolt with excellent cold forgeability by subjecting a medium carbon manganese steel rod to isothermal transformation process. This technique particularly focuses on decreasing strength unevenness of a steel material when hot rolling as well as decreasing deformation resistance during bolt manufacturing, thereby achieving manufacture of bolts with a tensile strength in the order of 1000 MPa. However, since not introducing a process for making the effect of hydrogen in the steel harmless, Patent Document 5 does not cover bolts with a tensile strength of 1200 MPa or more.